With the installation of energy storage systems most people look at the battery and concentrate on that but in reality the battery inverter is an integral part of any energy storage system and will dictate much of the energy storage systems operations.
Often when an energy storage system isn’t working properly the fault lies with the inverter and not the battery.
Inverter power is measured in kVA or volt amps.
The battery inverter takes power from the solar PV array (solar inverter or MPPT) and battery which can be of different voltages and converts it to 240V for household of business use.
Solar inverters are designed to work with solar PV arrays and battery inverters are designed to work with energy storage batteries. Hybrid inverters are also coming onto the market that combine both solar PV and battery conversions.
The battery inverter dictates;
- how much power can be used at any point in time and this has 3 components,
- rated power,
- surge power and
- operating power.
- These may be all the same or may be completely different.
- Some inverters can start a battery that can go to zero charge with no degradation (eg. salt batteries and bromide flow batteries).
- Some inverters have some storage capacity if the grid goes down.
- Inverters also operate at different voltages, i.e. 12V, 24V, 48V, 600V, 1000V.
- Inverters can be single phase or 3 phase.
When deciding on a particular inverter for your needs you need to look at all these variables, including which inverters are recommended for the battery technology you are considering.
Many appliances, particularly those that have electric motors like water pumps or refrigerators have high initial surge currents and much lower operating currents. Other appliances like kettles have a continuous high power draw, although are only used for short periods of time.
You need to look at power ratings of all appliances that will need to run off the energy storage system and add up all continuous power needs and surges and what will be running concurrently. You may only want certain circuits to run or a complete household or business (similar to an off grid situation).
Some inverters are better suited to particular battery technologies and less suited to others and most battery technologies will have particular inverters that they are recommended to be used with.
Batteries will often work correctly with certain inverters but are not warranted by that particular inverter company. This generally means the inverter company hasn’t done their own testing.
Generally inverter companies have a certain amount of money allocated to test and configure communications between their inverters and different battery technologies. Naturally they tend to allocate these resources to larger companies that they believe will sell more batteries into the market. This doesn’t necessarily mean the batteries are better quality or safer just that the battery companies are better at marketing.
Until a few years ago the only readily available batteries were lead acid batteries which generally operate at a recommended depth of discharge of 30 – 50% (70 – 50% capacity remaining in the battery). If a lead acid battery totally discharged it would be considered as ending its life and inverters wouldn’t restart them. As a result, many inverters won’t start a battery that has no charge in it.
Some batteries can be completely discharged without damaging the battery (eg. salt batteries and bromide flow batteries) and these batteries don’t work well with inverters that won’t start a battery with no charge, (there are work arounds that can be incorporated into a system design to overcome this problem).
Other batteries can be discharged to 80 – 90% SOC (state of charge) with no damage to the battery and similarly some older lead acid inverters have difficulty with these parameters and will switch off if they perceive the SOC getting too low.
Newer batteries, particularly those with BMS’s (battery operating systems) can have problems with a number of inverters if the communications are not set up correctly. With older inverters and inverters designed for batteries without a BMS the SOC of the battery is calculated using voltages. Usually a shunt is installed to measure what goes into and out of the battery and to estimate SOC. The BMS of a battery is designed to accurately measure many variables, including SOC of the battery and this information needs to be transferred to the inverter. If the SOC isn’t relayed correctly to the inverter the battery may have only 20 – 30% SOC and the inverter may think it has 100% SOC and will stop charging the battery, meaning the battery can go flat, as it is not being charged, not because there is a problem with the battery but because the communications between the battery and inverter are not set up correctly.
The inverter is often the component of any system that fails the fastest and like anything, generally you get what you pay for and the more expensive inverters will be more robust and will generally last longer. The cheaper inverters on the market will have limited capabilities and often a shorter lifespan.
These are just some of the questions you need to think about when installing a battery energy storage system and want to make sure the battery system is designed for your needs. If you aren’t asked these questions you need to ask if the salesman / installer is trying to find out the best system for your needs or just trying to sell you the system they want to sell you.
If you would like to know more about getting a safe, reliable and recyclable battery energy storage system for your own home, business or micro-grid to increase your energy independence visit us at http://quantum.GridEdge.com.au. We have a number of different sized systems that can cater to your budget and household needs.